1. Field of the Invention
The invention in general relates to optical components for display screens, in particular for liquid crystal monitors. In particular, the invention relates to optical components for displays with backlighting.
2. Description of Related Art
Liquid crystal displays are not self-illuminating and have to be illuminated from the back in order to obtain an illuminated image corresponding to other types of displays, such as cathode ray tubes, plasma display screens, or OLED displays. Displays, in particular those of high-resolution LCD television sets with high image quality, therefore require a so-called backlight system, in which, typically, light of white LEDs is in-coupled laterally at one or more edges indirectly or, from the back, directly into a light guide plate or diffuser plate. Such a system is referred to as edge lighting (edgelit backlight unit) or as backlighting (direct backlight unit). Traditionally, in this case, a plastic plate is used, for example one made of especially translucent PMMA (polymethyl methacrylate) or made of other transparent polymers that can be fabricated especially inexpensively and in high purity without selective absorption in the visible wavelength region. However, these materials often exhibit degradation phenomena due to undesired moisture uptake (incorporation of water molecules from the ambient humidity) and, when subjected to permanent light irradiation, can become brittle. This, in turn, is detrimental to the image quality and service life.
With increasing display screen diagonal dimensions—which tend to be greater than 55 inches up to, at present, generally at most 70-inch format—or else when individual displays are combined to form very large video walls, this material has a drawback, because it exhibits a very high thermal expansion coefficient, which is a multiple (>15 times greater) than that of the display glass, and is also especially susceptible to high local thermal load due to the heat input of the many light-emitting diodes (e.g., up to approximately 1500 LEDs in the case of direct in-coupling).
On account of the high thermal expansion coefficient as well as moisture-dependent expansion of the light guide plates made of plastic, compensatory spaces, in particular so-called spacer gaps, must be provided in liquid crystal monitors. This leads, on the one hand, to relatively wide frames of devices, but also to augmented depth (thickness) of devices, because flat components having different thermal expansion coefficients must be installed next to one another with sufficient spacing. In addition, plastic exhibits a low stability, so that, typically, additional structural components are required. Liquid crystal display screens therefore have a minimum thickness of about 30 millimeters with current technology.
For the production of liquid crystal TV sets that are as thin as possible and as light as possible, therefore, the hitherto conventional use of light guide plates made of plastic is a disadvantage.
US 2014/0043852 A1 and US 2014/0146267 A1 describe design solutions that are intended to overcome these disadvantages. The solutions shown therein are elaborate, however, and are only partially successful and, above all, they do not overcome the drawback in principle of PMMA light guides, namely, the necessary thickness thereof.
End consumers meanwhile desire television screens with a depth (thickness) of only a few millimeters. However, this is not technically possible if the PMMA light guide plate itself already exhibits a thickness of at least 3.5 mm in practice and additionally requires spacer gaps.
Although televisions or displays that operate by means of OLED technology attain very small thicknesses, they have the drawback within the production process that pixel flaws reduce the performance and that, particularly in the case of very large display screen diagonals of greater than 55 inches, these devices are very expensive. Beyond this, they need, in addition, a reinforcement element in order to ensure the requisite rigidity in day-to-day use.
Therefore, it would be advantageous to employ a glass as material for the light guide instead of the generally used PMMA plastic.
Described in WO 2015/033866 is an apparatus with an edgelit backlight unit and a light-emitting surface, said apparatus comprising a glass plate. However, the described features are not adequate to achieve a good transmission equivalent to that of PMMA. The best values described are just 83% or greater for a light-path length of only 100 mm.
US 2014/0152914 A1 describes a glass with high transparency for a touch display screen, which detects the finger position by means of frustrated total reflection. To this end, light in the wavelength range of 750-2500 nm, that is, in the infrared spectral region, is used. However, the samples show strong fluctuations of the absorption coefficient, in particular in the visible wavelength range of 400-800 nm. In this wavelength range, the maximum absorption coefficient is at least twice the minimum absorption coefficient. In the exemplary embodiment illustrated in FIG. 9, the absorption coefficient is even 0.00021 mm−1 at approximately 470 nm and 0.00089 mm−1 at approximately 430 nm. However, strong fluctuations in the absorption coefficient are not suitable for light guide plates. Desired instead are low, uniformly running absorption coefficients that result in a uniform transmission curve in the visible wavelength region.
WO 2015/011040 A1, WO 2015/011041 A1, WO 2015/011042 A1, WO 2015/011043 A1, WO 2015/011044 A1, and WO 2015/071456 A1 also relate to glass plates with high transmission in the infrared region, which can be employed in touch display screens, wherein, by means of the technology of so-called planar scatter detection (PSD) or by means of the frustrated total reflection, the position of objects on the surface is determined. It is known that Fe2+ and Fe3+ give rise to absorption bands with band maxima at 380 nm (relatively low absorption) and 1050 nm (relatively strong absorption), which can be influenced by oxidizing substances. The mentioned applications describe how, in the case of a tolerated, relatively high iron content (Fe2O3), it is possible to achieve a high transmission in the infrared region by targeted addition of chromium (Cr). However, these teachings cannot contribute to the optimization of light guide plates for the visible wavelength region.